1. Field of the Invention
The invention relates to a crankshaft production method and production apparatus for producing a crankshaft from a shaft blank.
2. Description of the Related Art
Japanese Patent Application Publication No. 49-106949 (JP-A-49-106949) describes, as a conventional technique of the aforementioned type, a technique for producing a crankshaft by the so-called “eccentric swaging-forging” by which the upper and lower end portions of a rod material (shaft blank) are pressed, while an intermediate portion of the shaft blank is caused to slide in the direction perpendicular to the axial line, and the shaft blank is swaged, while being budded. More specifically, as shown in a schematic cross-sectional view in FIG. 58, the production apparatus includes an upper block 61 provided movably in the vertical direction, a lower block 62 that is provided immovably in a transverse direction, and an intermediate block 63 that is provided in an intermediate position between the upper and lower blocks 61, 62. An intermediate portion of a shaft blank 64 is gripped by the intermediate block 63, swaging portions are formed above and below the intermediate portion, and the swaging portions are then buckled by applying a pressure and lowering the upper block 61 and also sliding the intermediate block 63 in the transverse direction, while lowering the intermediate block, in connection with the aforementioned operation. As a result, the swaging portions are pressurized between the upper block 61 and the intermediate block 63 and between the intermediate block 63 and the lower block 62.
Shaping a crankshaft in a cold forging mode by the above-described “eccentric swaging-forging” enables net shaping of the crankshaft and makes it possible to expect a reduction of thermal strains, a decrease in the number of machining steps necessary to produce the crankshaft, and a reduction of crankshaft production cost. In this case, in order to increase the variation of engine exhaust gas amount, it is necessary to vary the eccentricity amount of a pin portion, that is, the offset amount between a journal portion and a pin portion, in the production of a crankshaft by the “eccentric swaging-forging”. Increasing the eccentric amount of the pin portion is particularly technologically difficult.
With the technique described in JP-A-49-106949, the shaft blank has to be compressed in the axial direction, while applying a force that induces sliding in the intermediate portion of the shaft blank, in order to control the buckling direction of swaging portions. In this case, buckling of the shaft can be ahead of swaging and shear cracking can occur in the shaft blank in the buckling process. Further, where the offset amount between the journal portion and pin portion of the crankshaft increases to ensure the variation of the engine exhaust gas amount, shear cracking can occur in the shaft blank in the latter half of swaging. FIG. 59 is an enlarged cross-sectional view of the zone inside a dash line circle S9 in FIG. 58. As shown in FIG. 59, a clearance C1 of for example about “5 to 20 μm” is required between the upper end portion of the shaft blank 64 and a receiving orifice 61a of the upper block 61. However, after the shaping of the crankshaft has been started, as shown in FIG. 60, the shaft blank 64 shifts to one side through the clearance C1. Therefore, a load in the vertical direction cannot be efficiently applied as a compressive force to the shaft blank 64. As a result, where the eccentricity amount of the shaft blank 64 increases, the tensile stress of the arm portion of the crankshaft increases and a crack 66 can occur in a shaped article 65, as shown in a cross-sectional view in FIG. 61.